The Leupold Standard (STD) Scope Bases are the most versatile of any scope mounting system. The rear ring is secured by windage adjustment screws, allowing the scope's internal adjustments to be reserved for precise sight-in. Additionally, shims can be used for gross elevation adjustment, reserving the scope's elevation adjustment for bullet drop compensation dial applications. The STD Mount Base fits left and right-handed actions and provides maximum accessibility to the breech area of your rifle. Machined steel construction. Compatible with STD rings. Limited lifetime warranty.